Compression technology
Jun 1, 2008 12:00 PM, BY CARL FURGUSSON
Arquiva’s OB truck fleet uses MPEG compression to make efficient use of back-haul circuits.
Over the past three years, there has been a high level of take up of HDTV over multiple platforms. MPEG-4 AVC has largely replaced MPEG-2 in the distribution of HDTV via satellite to the home. IPTV has also used HD as a differentiator in its market entry strategy. In addition, several local channels in the United States are now gathering and transmitting news programming in HD. These factors, including the increasing international adoption of HD, have led to an increased demand for HD content.
With current forecasted consumer demands pointing toward even more bandwidth-hungry HD applications and screen capabilities now predominantly achieving full resolution, the requirement for multiple HD formats will grow. Operators are seeking a solution that enables them to acquire high-definition content across their current networks without significant change in costs.
MPEG-4 AVC is increasingly being seen as enabling technology that can meet these goals. In previous applications, it has been applied at low bit rate implementations whereas in the future, it will penetrate contribution networks.
Technology
Figure 1. Bit rate saving of MPEG-4 AVC over MPEG-2
Click to enlarge
It was initially assumed that the major benefits of MPEG-4 AVC were to be found at the low bit rate end of the spectrum. However, recent research has shown that there are significant gains to be found at higher bit rates.
Figure 1 shows the major savings available below 10Mb/s. The region of least gain appears around 20Mb/s, and there are increasing gains above the 20Mb/s operating point.
The coding gain of MPEG-4 AVC can be attributed to a number of new coding tools that are not available in MPEG-2. These include:
- nine 4 × 4 intra-prediction modes;
- four 16 × 16 intra-prediction modes;
- seven inter-prediction modes from 16 × 16 down to 4 × 4 block sizes;
- a quarter pel motion compensation;
- advanced bidirectional pictures;
- motion compensation from outside of the picture;
- multiple reference pictures;
- integer transformation;
- in-loop deblocking filter; and
- context-based adaptive binary arithmetic coding (CABAC).
The importance of 4:2:2 video compression
Traditionally, contribution applications have demanded 4:2:2 MPEG-2 video compression. This prevents the need to repeatedly down-sample and up-sample the chroma channels. Mismatches in spatial positioning of the chroma filters and/or soft filter roll-offs can significantly degrade the chroma quality.
Arquiva’s master control room at Crawley Court manages contribution and distribution networks for UK operations.
In addition, repeated concatenation of video encoding processes can impede compression performance through the later stages in the delivery chain of content reaching the consumer. Therefore, preserving optimum video quality is desirable from the very first stage of content acquisition.
Furthermore, 10-bit 4:2:2 is the most commonly used studio and production format. Therefore, from a compression efficiency prospective, it is important to know at what bit rate the picture quality of 10-bit 4:2:2 would be noticeably better than 8-bit 4:2:2.
Levels, profiles and operating points
ISO/IEC 14496-10, otherwise known as MPEG-4 AVC, defines a number of profiles applicable to video conferencing, broadcast and streaming applications:
- The Baseline Profile is mainly intended for video conferencing and streaming to mobile devices. Its simplistic applications mean that it does not support bidirectional frames, interlace or CABAC entropy encoding.
- The Main Profile is most widely used for HD and SD broadcasting applications. It allows bidirectionally predicted frames with two direct modes (spatial and temporal) and weighted predictions. Furthermore, it supports all interlace coding tools including picture-adaptive field/frame coding (PAFF) and macroblock-adaptive field/frame coding (MBAFF) as well as CABAC.
- The coding tools of MPEG-4 profiles that go beyond Main Profile are summarized as Fidelity Range Extensions. In particular, the High Profile allows adaptive 8 × 8 integer transforms, intra 8 × 8 predictions modes and scaling lists. The High 10 Profile allows coding of 4:2:0 video signals with 10-bit accuracy, and the High 4:2:2 Profile allows coding of 4:2:2 video signals with 8- or 10-bit accuracy.
Here we are concentrating on high-definition and therefore the High 10 Profile and the High 4:2:2 Profile in particular. Assuming that there is no debate over whether 4:2:2 is an absolute requirement for contribution networks, then the focus will shift to the choice between 8- or 10-bit encoding.
Comparison between 8- and 10-bit encoding
An important question to answer when comparing 8-bit with 10-bit video signals is to decide which assessment method should be used for the comparison. Because we are dealing with relatively high picture qualities, objective methods, such as structural similarity and PSNR, are preferable to subjective assessment methods. Subjective assessment tends to become less accurate the higher the picture quality because distortions are less obvious.
In order to compare YCbCr PSNR results for 4:2:0 with those for 4:2:2, a method to combine individual PSNR figures for Y, Cb and Cr into a single number has to be defined. Because chrominance distortion is far less visible than luminance distortion, a weighted sum of 0.8Y + 0.1Cr + 0.1Cb is used. The same weighting factors have also been independently proposed, even though it is a relatively arbitrary definition that has not been verified by extensive subjective tests. Therefore, the comparison between 4:2:0 and 4:2:2 has to be interpreted with some caution. Nevertheless, such a definition allows us to investigate the trade-off between chroma resolution and picture distortion on different test sequences.
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